The distance over which a signal can be successfully transmitted through an optical fibre is limited by two distinct types of loss which occur in glasses generally: losses arising from absorption, e.g. in electronic transitions or vibrational transitions; and losses due to Rayleigh scattering (a phenomenon resulting from inhomogeneities). The latter type of loss is a theoretically inevitable one, although it becomes far less significant at longer wavelengths. If a glass can be formulated so that the absorption is very low at long wavelengths, very low loss (absorption plus Rayleigh scattering) can be achieved in communications at such wavelengths.
In germanosilicate and borosilicate glass fibres, the absorption spectrum is such that they have minimum loss in the range of 0.8-1.2 .mu.m. Such fibres are less suitable for long than for short distance communication, e.g. between computers. An absorption peak at about 950 nm may be attributed to OH vibrational absorption, and various solutions have been proposed to reduce loss at or around this wavelength, by the passage of gases through the glass melt, such gases being CO/CO.sub.2 (GB-A-No.1507712), dry oxygen (GB-A-No. 2033373), chlorides such as SiCl.sub.4 (JP-A-No.56-149332) and fluorine-containing gases, e.g. C.sub.3 F.sub.8 and F.sub.2, as drying agents (EP-A-No. 0103441).
Loss may also be attributable to impurities in the glass constituents and therefore in the glass. One class of impurities is the transition metal elements, but the need to reduce the loss caused by such compounds is dependent on the relationship between the wavelength at which they absorb and the transmission wavelength; if these wavelengths are sufficiently different, the impurities can be tolerated.
GB-A-No. 1507712 (see above) discloses that the effect of Fe and Cu impurities in oxide glasses may be reduced, not by removing the compounds as such but by changing the oxidation states of the metals, to Fe(III) and Cu(I), respectively. As.sub.2 O.sub.3 and other redox buffering agents are proposed for use with the reducing gas CO.
By these various means, it has been sought, in theory or practice, to reduce the absorption losses inherent in optical fibres. A complementary or alternative procedure has been to reduce Rayleigh scattering by formulating glasses which can be operated at longer wavelengths. Silica glasses have loss minima at 1.3 and 1.55 .mu.m and are already being manufactured and used for long distance communication. More recently, fluoride glasses have been prepared which, it is suggested, might be operated in the 2 to 10 .mu.m range. It seems more likely that, in the immediate future, the operational range for such fibres will be of the order of 3 .mu.m. As operational wavelengths increase, there is corresponding difficulty in formulating suitable lasers.
Preferred multi-component fluoride glass compositions are ZrF.sub.4 -based. Examples of components of such compositions are, in addition to ZrF.sub.4 (or HfF.sub.4), BaF.sub.2, LaF.sub.3, GdF.sub.3, AlF.sub.3 and NaF or LiF. Particular compositions, and a general background to the formulation of non-silica-based infra-red fibres, are given by Miyashita et al., IEEE Journal of Quantum Electronics, QE-18, No. 10 (Oct. 1982).
Some multi-component halide glasses are exemplified in Table 1 of the article by Miyashita et al. Suitable compositions can be determined from phase diagrams of the type given for a ZrF.sub.4 -BaF.sub.2 -GdF.sub.3 composition in FIG. 3 of the article by Miyashita et al.
Robinson et al, Mat. Res. Bull. 15 (1980) 735-742, report the use of CCl.sub.4 as a drying agent in fluoride glasses. Halogen and halogen-containing compounds are generally known for use in reactive atmosphere processing.
Tran et al, Sixth Topical Meeting on Optical Fiber Communication (28 Feb.-2 Mar. 1983), New Orleans, Digest of Technical Papers, page 7, disclose reactive atmosphere processing of ZrF.sub.4 -based glasses using SF.sub.6, HF, CCl.sub.4, CF.sub.4 and NH.sub.4 HF.sub.2, to effect OH removal.
Bansal et al, J.A. Ceram. Soc. 66(4) (1983) 233, disclose reactive atmosphere processing of ZrF.sub.4 -based glasses under Cl.sub.2, in a study of crystallisation kinetics.
Lecoq et al, Verres et Refractaires 34(3) (1980) 333-342, describe the role of Al as a stabiliser in ZrF4-based glasses. The glass composition was made by melting the constituents under ambient air, "which causes partial hydrolysis of the material at the moment of casting, but without preventing the production of glass if the amount of the sample is sufficient (&gt;5 g)". Air, of course, includes water and dust; both can have undesirable effects, by reaction or physical incorporation, on the properties of, say, optical fibre.
Almeida et al, J. Non-Cryst. Solids 56 (1983) 63-68, disclose that a pronounced effect on the IR absorption edge of bulk ZrF.sub.4 -based glasses is observed if oxide impurities are present, and that oxygen atoms tend to occupy bridging positions in the ZrF.sub.4 chain-like glass skeleton.
Fe(II) and Cr(III), for example, have been observed in fluoride glass compositions. It is reasonable to infer that these are the only oxidation states in which these transition metals can exist, stably, in an fluoride glass matrix.
It is well known that oxygen must be excluded when preparing fluorides. It appears also that oxides and oxygen atoms should not be introduced into fluoride glass compositions. However, there is as yet no satisfactory solution to the problem of reducing absorption losses in such compositions, without going to economically unacceptable lengths to purify the constituent materials.